1. Field of the Invention
This invention relates to a semiconductor fabrication process, and more particularly to an alignment system for performing an alignment process on a wafer.
2. Description of Related Art
Among the processes for fabricating a semiconductor device, photolithography is one of the most critical processes since it is directly related to the accuracy of transferring a pattern on to the desired region. Generally, the fabrication of a semiconductor device requires about 10 to 18 masks, that is, performing the same number of photolithography processes.
Before a pattern is transferred onto a desired surface, an alignment process has to be done to ensure the accuracy of patterning. Normally at least two alignment marks are formed on the surface of a wafer to be patterned in order to set the wafer on a correct position. However, various fabricating processes performed on the wafer, such as chemical mechanical polishing, cause damage on the alignment marks, and that further results in a poor alignment and an erroneous pattern transfer.
Referring to FIG. 1 is a schematic diagram showing a conventional method for aligning a wafer 14. Referring to FIG. 1, the wafer 14, which has a top surface 13 and a back surface 15, is placed on a wafer stage 12, wherein the top surface 13 of the wafer 14, contains two alignment marks W1 and W2. The wafer stage 12 is located on a fixed base 10.
The alignment process starts with emitting a guiding ray from an illumination case 16. The guiding ray is firstly reflected by mirrors 18 and 20, and then the reflected guiding ray falls onto one of the alignment marks W1 and W2. The guiding ray is then reflected to a reticle 32 through a projection lens 30 by the wafer, wherein the reticle 32 contains marks M1 and M2. Two alignment blocks 34 and 36 are used to confirm the condition of a valid alignment is obtained. The condition of a valid alignment includes that the reflected guiding ray strikes on the marks M1 and M2.
Because most fabrication processes physically change the top surface 13 of the wafer 14, the alignment marks W1 and W2 are deformed and maculed. The physical damages on the alignment marks lead to a poor alignment, poor pattern transfer, or even resulting in abandoning a wafer.